Re: [PATCH 1/2] vmscan: fix initial shrinker size handling

[Johannes Weiner]

On Tue, Sep 13, 2011 at 08:38:36PM +0200, Johannes Weiner wrote:
> On Tue, Aug 23, 2011 at 10:00:54AM +1000, Dave Chinner wrote:
> > On Mon, Aug 22, 2011 at 04:38:21PM -0700, Andrew Morton wrote:
> > > On Tue, 23 Aug 2011 09:22:57 +1000
> > > Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > > 
> > > > On Mon, Aug 22, 2011 at 02:17:21PM +0300, Konstantin Khlebnikov wrote:
> > > > > Shrinker function can returns -1, it means it cannot do anything without a risk of deadlock.
> > > > > For example prune_super() do this if it cannot grab superblock refrence, even if nr_to_scan=0.
> > > > > Currenly we interpret this like ULONG_MAX size shrinker, evaluate total_scan according this,
> > > > > and next time this shrinker can get really big pressure. Let's skip such shrinkers instead.
> > > > > 
> > > > > Also make total_scan signed, otherwise check (total_scan < 0) below never works.
> > > > 
> > > > I've got a patch set I am going to post out today that makes this
> > > > irrelevant.
> > > 
> > > Well, how serious is the bug?  If it's a non-issue then we can leave
> > > the fix until 3.1.  If it's a non-non-issue then we'd need a minimal
> > > patch to fix up 3.1 and 3.0.x.
> > 
> > I'm pretty sure it's a non-issue. I'm pretty sure all of the
> > shrinkers return a count >= 0 rather than -1 when passed nr_to_scan
> > == 0 (i.e.  they skip the GFP_NOFS checking), so getting a max_pass
> > of -1 isn't going to happen very often....
> 
> Except for the case which Konstantin laid out, grabbing the super
> block reference.  How likely is that?  And why isn't once enough to
> build up quite a high number?
> 
> > And with total_scan being unsigned, the negative check is followed
> > by a "if (total_scan > max_pass * 2)" check, which will catch
> > numbers that would have gone negative anyway because max_pass won't
> > be negative....
> 
>                 total_scan = nr;
>                 max_pass = do_shrinker_shrink(shrinker, shrink, 0);
>                 delta = (4 * nr_pages_scanned) / shrinker->seeks;
>                 delta *= max_pass;
>                 do_div(delta, lru_pages + 1);
>                 total_scan += delta;
> 
> max_pass, an unsigned long, is what the shrinker returned, so
> ULONG_MAX.  ULONG_MAX * 2 is ULONG_MAX - 1, still pretty big?
> 
> Even for high values of delta (lots of pages scanned, few lru pages
> left), it won't come nowhere near max_pass such that the product of
> the two is a reasonable number again.

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